1. Field of the Invention
The invention relates to a driving circuit, more particularly, to a light emitting diode (LED) backlight driving circuit with transformers for current balance in an LED driving field.
2. Description of Related Art
A backlight source of a conventional liquid crystal display (LCD) is mainly formed by cold cathode fluorescent lamps (CCFLs), though with development of light emitting diode (LED), since the LED has advantages of high light emitting efficiency, fast response speed, good color representation, long service life and none mercury, etc., in large panel applications such as an LCD TV, it has replaced the CCFLs and becomes a main backlight source.
In order to obtain enough brightness, a plurality of LEDs is connected in series for utilization. However, considering a specific power conversion and security, etc., the number of the LEDs of each LED string is generally limited, so that in an actual large size of LCD backlight application, the LEDs are generally connected in series to form LED strings first, and then the LED strings are connected in parallel to form an LED array. To unify the brightness of the whole LCD screen, each of the LED strings has to provide a same brightness. According to a characteristic of the LED, the brightness of the LED is proportional to a driving current thereof, and a small variation of a voltage difference between two ends of the LED may lead to a large variation of a current flowing through the LED. Therefore, to produce the constant brightness, a constant current control has to be performed to each LED string of the LED backlight source.
A typical structure of LED backlight source (shown in FIG. 1) is generally composed of three stages of circuits, where a first stage is a power factor correction (PFC) circuit, a second stage is a direct current (DC)-DC isolation converter, and a third state is an LED driving circuit. The conventional LED driving circuit is as that shown in FIG. 2. The LED driving circuit is used for driving the LED strings, and performing a current balance/equalization to each of the LED strings, so as to make each of the LCD strings have the same brightness.
An LED current balance circuit is generally implemented by active devices such as operational amplifiers, transistors, metal oxide semiconductor field effect transistors (MOSFETs), or a pulse width modulation (PWM) controller integrated by the above devices. When the LED current balance circuit composed of the active devices (shown in FIG. 3) performs current balance/equalization, a device conduction loss is huge, so that a heat dissipation problem of the whole backlight system is hard to be resolved. Moreover, in the active current balance circuit, each LED string requires a specific current balance/equalization control circuit to individually control a current of each LED string, which may increase system complexity and cost and reduce system efficiency, so that the active current balance/equalization method is not applicable when the number of the LED strings is large.
In order to overcome the defect of the active current balance circuit, passive current balance circuits composed of passive devices such as capacitors, inductors are developed, which include 1. a capacitor current balance circuit, shown in FIG. 4(a), which is consisted of a capacitor and two anti-parallel LED strings, wherein the capacitor and the two anti-parallel LED strings are connected in series. An input of the circuit is a high-frequency alternating current (AC) signal, and a waveform of the current flowing through each LED string is an AC half-wave, so that a maximum duty cycle of each LED string is only 50%, and only the two anti-parallel LED strings connected in series with the same capacitor can implement a good current balance/equalization effect, and the other LED strings cannot implement the current balance/equalization; and 2. a transformer current balance circuit, shown in FIG. 4(b), in which a primary side and a secondary side of a transformer are respectively connected to two LED strings in series to implement the current balance/equalization, though such method can only be applied in applications when the number of the LED strings is an even number.